1. Field of the Invention
The invention concerns memory cards and, more particularly, those used as a means of prepayment in installations for the dispensing of products and/or services.
2. Description of the Prior Art
A typical example of a memory card to which the present invention can be applied is the prepaid phone card which is purchased for a certain monetary value. This value is memorized directly or indirectly in the card and is decremented as and when it is used. The invention, however, can also be used in other situations where accounting units are recorded in the card and are decremented or incremented as and when they are used. These units may or may not have a monetary value. In the rest of this description, we shall consistently use the example of the prepaid telephone card for it is the most eloquent example that could be used to explain the advantages of the invention.
In a previous embodiment of a prepaid card, a non-volatile electrically programmable and electrically non-erasable memory is used as a means of storage of the prepaid value.
In this embodiment, the non-volative electrically programmable memory (EPROM) contains P memory cells. Each cell corresponds to an elementary accounting unit of the commercial value. The cells are initially blank. During use, a card reader associated with the dispensing of products or services gives successive pulses for programming the P cells one after the other, as and when the dispensing takes place. The residual value of the card corresponds to the number of cells that have not yet been programmed. When all the cells are programmed, the prepaid credit is exhausted.
In another embodiment, the memory is electrically erasable (EEPROM). Attention will be paid more particularly to this latter example which more clearly highlights the advantages of the invention. The memory has P cells, each corresponding to an accounting unit. There is furthermore provision for a counter, a register containing a fixed number Z corresponding to a total commercial value Z.times.P of the card expressed in accounting units, and a comparator to compare the content of the counter and that of the register. When the P cells of the memory have been programmed, the counter is incremented by one unit, and the cells are all erased simultaneously, then again programmed successively, one after the other. The content of the counter gets incremented after each consumption of P accounting units. When the content of the counter reaches the value specified in the register, the comparator delivers a no more credit signal which prevents the continued use of the card. Thus, through the fact that the card is electrically erasable, a card is made having a credited value of Z.times.P although the memory has only P cells.
Of course, the counter is incremented by a circuit internal to the card. It is not accessible at the external terminals of the card as, in this case, fraudulent activity would become possible.
In practice, it is possible to conceive of cards containing, for example, a 50-cell memory (P=50), and cards of 50, 100, 150, etc. accounting units could be commercialised.
The erasable character of the memory is used to preserve a limited number of memory cells (P cells). At the same time, broad possibilities of choice will be retained as regards the commercial value of the card (P.times.Z). The manufacturer of the integrated circuit will have only one circuit to design, and all he has to do to assign one commercial value or another to the card is to use program the content of a register (in practice a read-only memory) defining the number Z.
The present invention proposes mainly to use the erasable character of the memory to make the card rechargeable, i.e. so that the user who has exhausted his credit can purchase an additional quantity of accounting units without changing the card.
Another aim of the present invention is that when the card contains only a small number of accounting units, these units should not be lost to the user, even when he recharges the card.